Video display device

ABSTRACT

A histogram generation portion calculates a prescribed feature value of a pixel and the maximum feature value for the pixel, for each pixel of one frame of an input video signal. Then, the ratio of the feature value of the pixel to the maximum feature value is calculated for each pixel as an index of color brightness, and a histogram generated wherein the numbers of pixels are integrated by index value. An enhancement processing portion applies gain and carries out an enhancement for pixel values wherein the index in the histogram which the histogram generation portion generates is greater than or equal to a prescribed threshold. The feature value is treated as a luminosity (L*) which is defined with CIELAB chroma space. Additionally, the feature value may be data which has the maximum tone value among pixel RGB data, or may be a luminosity value (Y) of the pixel.

TECHNICAL FIELD

The present invention relates to a video display device, and moreparticularly to a video display device provided with an enhancementfunction for improving image quality of a displayed video.

BACKGROUND OF THE INVENTION

In a video display device, an enhancement function for improving imagequality of a displayed video is known. In a case where the enhancementfunction is executed, generally, when a maximum value of a tone isdetected for each frame of a video signal and the level of the maximumvalue is low, gain is applied to a video signal in a part with a hightone for emphasis. In addition, when a minimum value of atone of a videosignal is detected and the minimum value is high, compression gain isapplied to a video signal in a part with a low tone to lower the tone.Such an enhancement function is used, so that a signal range of a videosignal is broadened and contrast feeling of a displayed image isincreased, thus improving image quality.

For example, Patent Document 1 discloses a liquid crystal display deviceautomatically controlling contrast so that, along with the adjustment ofluminance of a backlight, brightness/darkness of an image also comesclose to the state before the adjustment. In this liquid crystal displaydevice, an operator turns on/off a light source of a backlight device,thereby allowing to change luminance of an image and aim for powersaving, an enhancement function works when the luminance is changed,contrast of a displayed image is controlled to be suited to theluminance, and even though the luminance of the backlight device islowered, it is possible to obtain almost same level ofbrightness/darkness of the image as before the luminance is lowered.

PRIOR ART DOCUMENT Patent Documents

-   Patent Document 1: Japanese Laid-Open Patent Publication No. 9-80378

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In a conventional enhancement function, processing is performed suchthat a maximum value or a minimum value of a pixel value of a videosignal is checked to emphasize and raise a high part of a tone orcompress and reduce a low part of a tone. Such enhancement processing isperformed by checking a pixel value of an image, thus does notnecessarily improve video quality sufficiently to be coincident with ahuman visual performance. For example, among object colors at the timeof shooting various objects, there exists a color which is luminous inappearance although luminance thereof is low as a video signal.

In addition, in the case of considering on an xy chromaticity diagram,it is possible to represent all colors by chromaticity identified on atwo-dimensional plane by x and y, and Y indicating lightness in avertical direction to the two-dimensional plane. In a case where a colorfrom an object is only reflected light, the lightness Y at eachchromaticity is limited up to an identified value. A maximum oflightness at the time is a maximum lightness. Only with the reflectedlight, an upper limit of lightness of each chromaticity is the maximumlightness while in a case where an object itself is emitting light,lightness is allowed to have a value which is greater than or equal tothe maximum lightness. A color having the lightness which is greaterthan or equal to the maximum lightness is generally referred to as aluminescent color. It is generally difficult to represent theluminescent color on a video signal.

That is, there is sometimes a gap between a value of a video signal, andwhat is seen and felt. When an enhancement is carried out by detecting acolor which looks bright by human eyes, it is possible to obtain aneffect of further highlighting a color which is luminous on a screen toimprove image quality, however, enhancement processing based on such anidea has not been conventionally performed.

The present invention has been devised in view of circumstances asdescribed above and aims to provide a video display device capable ofimproving video quality in a state more suited to a human visualperformance by focusing on color brightness of a video signal andperforming enhancement processing for emphasizing and highlighting abright color more.

Means for Solving the Problem

To solve the above problems, a first technical means of the presentinvention is a video display device for enhancing and displaying aninput video signal based on a feature value of the input video signal,comprising: a histogram generation portion for generating a histogram ofthe feature value of the input video signal; and an enhancementprocessing portion for regarding a top region in a prescribed range ofthe histogram generated by the histogram generation portion as aluminescent color and enhancing a pixel value of the luminescent color;and a compression gain processing portion for uniformly decreasing gainover the entire pixels of one frame subjected to enhancement processingafter the enhancement processing portion performs the enhancementprocessing, wherein the histogram generation portion calculates, foreach pixel of one frame of an input video signal, the feature value of apixel and a possible maximum of the feature value of the pixel,calculates, for each pixel, a ratio of the feature value of the pixel tothe maximum of the feature value as an index value of color brightness,and generates a histogram in which the number of pixels is integratedaccording to the index value, the enhancement processing portion regardsa pixel in which the index value in the histogram generated by thehistogram generation portion is greater than or equal to a prescribedthreshold as the top region and carries out an enhancement by givinggain-up for a pixel value in the top region, the enhancement processingportion integrates, at the time of enhancing the pixel which is greaterthan or equal to the threshold, the number of pixels from a pixel havinga highest feature value in a direction of low feature value of thehistogram, applies gain-up in which the M-th (M is a prescribed value)percentile pixel of the total number of pixels is displayed in adisplayable highest tone, and temporarily permits that the pixel valueexceeds the displayable highest tone by the applied gain-up, and thecompression gain processing portion performs, in a case where the pixelvalue exceeds the displayable highest tone even after the compressiongain processing portion applies gain-down for the temporarily permittedpixel value, soft clip so that the pixel exceeding the highest tone isoutput in a tone within the highest tone as well as that an output valuein a region from the highest tone up to a prescribed low tone graduallylowers in a curve.

A second technical means is the video display device of the firsttechnical means, wherein the enhancement processing portion determines,the threshold ‘thresh’ by ‘thresh’=A+Nσ (N is a constant) where theprescribed threshold is a ‘thresh’, an average value of the index valueof the histogram is A, and a standard deviation is σ.gram is A, and astandard deviation is σ.

A third technical means is the video display device of the firsttechnical means, wherein the enhancement processing portionexponentiates a logarithmic average of a luminance value of a pixel ofone frame of an input video signal to calculate a geometric averagevalue, and carries out the enhancement by the gain-up in a case wherethe calculated geometric average value is lower than a prescribed value.

A fourth technical means is the video display device of the thirdtechnical means, wherein the enhancement processing portion sets asecond prescribed value which is lower than the prescribed value of thegeometric average value, gradually increases gain which is applied asthe enhancement in order from the prescribed value up to the secondprescribed value in a region from the prescribed value to the secondprescribed value, and keeps the gain constant in a region where thegeometric average value is lower than the second prescribed value.

A fifth technical means is the video display device of the firsttechnical means, wherein the feature value of the pixel is luminance L*which is specified by CIELAB color space.

A sixth technical means is the video display device of the firsttechnical means, wherein the feature value of the pixel is dataincluding a maximum tone value among pixel RGB data.

A seventh technical means is the video display device of the firsttechnical means, wherein the feature value of the pixel is a luminancevalue Y of a pixel.

Effect of the Invention

According to the video display device of the present invention, it ispossible to improve video quality in a state more suited to a humanvisual performance by focusing on color brightness of a video signal andperforming enhancement processing for emphasizing and highlighting abright color more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a partial configuration example of avideo display device according to the present invention.

FIG. 2 is a diagram explaining luminescent color enhancement processingthat is executed in the video, display device of the present invention.

FIG. 3 is a diagram explaining a setting example of gain for performingenhancement processing.

FIG. 4 is a diagram showing a state where a CMI histogram in FIG. 2 isenhanced using calculated gain.

FIG. 5 is a diagram showing an example of a state before and aftercarrying out an enhancement with a tone curve.

FIG. 6 is a diagram showing a tone curve with RGBLUT created from RGBdata shown by the tone curve in FIG. 5.

FIG. 7 is a diagram showing a tone curve in a case where output of thetone curve in FIG. 5 is used as input in FIG. 6.

FIG. 8 is a diagram schematically showing a luminescent color boundarywhere CMI=100 in relation between saturation and lightness.

FIG. 9 is a diagram showing a histogram of a luminance and chromaticityof a real object or the like, and a histogram after standardizationthereof.

FIG. 10 is a diagram explaining a method of calculating CMI from abroadcast video signal to be displayed on the video display device.

FIG. 11 is a diagram showing an example of a CMI histogram.

FIG. 12 is a diagram showing a response curve of a human photoreceptorcell with respect to luminance.

FIG. 13 is a flowchart for explaining luminescent color enhancementprocessing in a first embodiment of the video display device accordingto the present invention.

FIG. 14 is a diagram explaining an optimal color in a pixel having RGBdata.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 is a block diagram showing a partial configuration example of avideo display device according to the present invention, where providedarea video signal processing portion 1, a histogram generation portion2, an enhancement processing portion 3 and a compression gain processingportion 4. The video display device includes, other than thesecomponents, a not shown display portion such as a liquid crystal panel,a backlight for illuminating the display portion and the like. In thevideo display device, signal processing for an audio signal and outputprocessing to a speaker and an audio output terminal are also performed.

The video signal processing portion 1 performs video signal processingas with conventional processing for inputting a video signal separatedfrom a broadcast signal and a video signal input from an externaldevice. For example, I/P conversion, noise reduction, scalingprocessing, γ adjustment, white balance adjustment and the like areappropriately executed. Moreover, contrast, a color tone and the likeare controlled based on a user setting value.

A video subjected to signal processing is input to the histogramgeneration portion 2 to create a histogram for each frame. The histogramgeneration portion 2 calculates, for each pixel of one frame of theinput video signal, a prescribed feature value related to colorbrightness of an input video signal and a possible maximum feature valueof each pixel while holding chromaticity, and calculates for each pixela ratio of a feature value of the pixel to the maximum feature value asan index value of color brightness. Then, a histogram is generated inwhich the number of pixels is integrated according to the index value.The above-described prescribed feature value is luminance L* specifiedby CIELAB color space. Further, in another example, the above-describedfeature value is data including a maximum tone value among pixel RGBdata. In still another example, the above-described feature value is aluminance value Y of a pixel.

The enhancement processing portion 3 applies gain (increases gain) andcarries out an enhancement for a tone of a pixel that is regarded as aluminescent color. The enhancement processing portion 3 calculates athreshold based on an average value A of the above-described index valueof the histogram and a standard deviation σ, and applies gain andcarries out an enhancement for a pixel value of a pixel having theabove-described index value which is greater than or equal to athreshold.

Further, the enhancement processing portion 3 raises a logarithmicaverage of a luminance value of a pixel of one frame of the input videosignal to calculate a geometric average value, and carries out anenhancement by applying gain in a case where the calculated geometricaverage value is lower than a prescribed value.

The enhanced video signal is input to the compression gain processingportion 4. After the enhancement processing portion 3 performsenhancement processing, the compression gain processing portion 4uniformly applies compression gain (decreases gain) over the entirepixels of one frame subjected to enhancement processing. A compressionamount is decided according to prescribed conditions of screen averageluminance and the like.

In an embodiment of a liquid crystal display device according to thepresent invention, a luminescent color is detected from a video signalto perform enhancement processing for the detected luminescent color.The luminescent color refers to a color with a prescribed level or moreof brightness on a lightness axis in a color represented on achromaticity diagram, and the enhancement processing is performed for avideo signal concerning a pixel of a color with a color brightness levelwhich is greater than or equal to a prescribed threshold. The thresholdin this case is a relative value that is defined according to ahistogram for each image of one frame, and enhancement processing isperformed for an image with a relatively prescribed level or more ofbrightness in images.

Enhancement processing is performed for a luminescent color, so that itis possible to perform enhancement processing for an actually brightcolor in appearance. Then, for a video with the enhanced luminescentcolor, compression gain is uniformly applied to lower a tone. Sincebrightness of the entire screen becomes higher in the case of holding anenhancing state, compression gain is applied again to lower thebrightness of the screen. Since enhancement processing is performed fora luminescent color, thereafter applying compression gain to the entirescreen, the processing in this case is equal to processing forredistributing luminance on the screen. Such processing allows a partwith a high luminescent color to be especially highlighted.

In luminescent color enhancement processing that is executed in thevideo display device according to the present invention, as describedabove, a luminescent color is detected from an input video signal toperform enhancement processing for emphasizing a tone for the videosignal of the luminescent color. In the processing, first, a geometricaverage value (GAve) and a CMI (color mode index) histogram are detectedfrom the input video signal. The CMI is one of the feature values forperforming enhancement processing according to the present invention.

The geometric average value is not an average of signal luminance but aluminance average value in which an average of luminance of a liquidcrystal panel is calculated as a value that is coincident with a visualperformance. Further, the CMI histogram is a histogram created based ona video signal and coincident with a human visual performance in whichthe number of pixels is integrated according to an index indicatingcolor brightness, which is different from a Y histogram or the like inwhich the number of pixels is integrated according to the luminancevalue Y of a video signal. Description will be given below for specificdefinitions and calculation methods of the geometric average value andCMI histogram.

FIG. 2 is a diagram explaining the luminescent color enhancementprocessing that is executed in the video display device of the presetinvention, showing an example of the CMI histogram that is generatedfrom an input video signal. When the CMI histogram is detected, anaverage value (Ave) and a standard deviation (σ) are calculated from theCMI histogram to be used for calculating a threshold, ‘thresh’, by‘thresh’=Ave+Nσ,where N is a prescribed constant. Other methods including at leastidentifying the top of the population may also be applied as thecalculation method of the threshold.

For example, assuming that the CMI histogram as shown in FIG. 2 isdetectable from one frame of an input video signal, a horizontal axis ofthe CMI histogram is CMI, and indicates the CMI of the brightest colorin all colors on a chromaticity diagram as 100. That is, colorbrightness is different according to tones of video signals, and the CMIis calculated from the tones to integrate the number of pixels. Avertical axis indicates the number of pixels (frequency) integrated bycolor brightness (CMI).

Additionally, the ‘thresh’ calculated as described above is applied tothe CMI histogram. The ‘thresh’ is used for determining a luminescentcolor from the CMI histogram, and a pixel with the CMI that is greaterthan the ‘thresh’ is defined as a pixel for displaying a luminescentcolor.

Then, among pixels with the CMI that is greater than the ‘thresh’ in theCMI histogram, pixels are accumulated in order from the highest CMI tothe lowest CMI to calculate gain for an enhancement so that the CMI ofthe M-th percentile pixel among all pixels becomes 100. The value ofgain thereof is a maximum value of gain (MAX) that is decided accordingto a geometric average value which will be described next.

FIG. 3 is a diagram explaining a setting example of gain for performingenhancement processing. Gain that is applied in the luminescent colorenhancement processing is calculated using the geometric average valuecalculated as described above and the maximum value of gain that iscalculated based on the M-th percentile pixel from the maximum value ofthe CMI.

As shown in FIG. 3, in the case of a video having a geometric averagevalue that is greater than or equal to a first value g1 of apredetermined geometric average value (GAve), an enhancement is notcarried out for the video (frame). A video with a high geometric averagevalue is a video which is entirely bright, and when this video iscompressed again after carrying out an enhancement, an impression that ascreen becomes dark by compression is greater than an impression that aluminescent color is highlighted by an enhancement. Thus, an enhancementis not carried out for the video having the geometric average value thatis greater than or equal to a prescribed level of the geometric averagevalue g1. That is, gain=1.0.

Moreover, between a first geometric average value g1 and a secondgeometric average value g1 that is lower than the former, gain isgradually increased in order from g1 to g2 from 1.0 up to the maximumvalue. The maximum value of gain (MAX) is a value calculated so that theabove-described CMI of the M-th percentile pixel becomes 100.

In a case where the geometric average value is smaller than g2, gainwith which the CMI of the M-th percentile pixel becomes 100 is applied.At the time, a pixel with the CMI larger than that of the M-thpercentile pixel may have the CMI that is larger than 100 as a result ofapplication of gain. In this case, the pixel with the CMI that is largerthan 100 is subjected to clipping in actual display, thus causing crush,thereby allowing no half-toning.

In this case, since the brightest level of a pixel is originallyilluminated with a high tone, it is less important to finely represent atone. On the other hand, for example, in the case of calculating gainbased on the highest value of the CMI, it is possible to performhalf-toning for all pixels, however, a pixel having a specifically highCMI in a video influences gain, and thus the gain has substantiallylittle effect in some cases. In the case of not allowing a certain levelof gain to be applied, a subsequent compression amount becomes small,and it is impossible to sufficiently attain an object of the presentinvention that a luminescent color is enhanced to increase imagequality. Thus, gain calculation is performed so as to allow crush ofpixels up to the M-th percentile in order to secure a certain amount ofgain. However, because of also having pixels with the CMI that islowered again by applying compression gain after carrying out anenhancement by applying gain, all pixels with the CMI which are largerthan M-th percentile are not necessarily subjected to clipping.Moreover, in actual processing, for an area which is crushed aftercompression, processing is performed for holding a tone as far aspossible by performing soft clip processing described below.

FIG. 4 is a diagram showing a state where the CMI histogram in FIG. 2 isenhanced using the gain calculated as described above. When gain for anenhancement is calculated in the above-described processing, the gain isapplied to carry out an enhancement. A target pixel for carrying out anenhancement is a pixel with the CMI which is greater than or equal tothe ‘thresh’, and enhanced by applying gain to a luminance L* componentin the CIELAB color space of the pixel. Thereafter, the color space isreturned to RGB color space. The amount of gain is calculated based onthe geometric average value of a video and maximum gain based on theCMI. That is, based on the ‘thresh’ calculated from the CMI histogram, apixel of a bright color is emphasized by carrying out an enhancement,while a pixel of a dark color is kept as-is.

At the time, in the state with the enhancement in FIG. 4, a pixel withCMI exceeding 100 also is not clipped and kept allowed. The value withCMI exceeding 100 is a value exceeding 255 of a tone value representedwith eight bits, for example, and thus is not allowed to be representedon a screen, however, at this time point, the CMI exceeding 100 isallowed because it is considered that there is a pixel with the CMIlowered again since compression gain is applied to the entire video atnext processing. The highest value of the CMI at the time of enhancementis obtained bygain X(Max−‘thresh’)+‘thresh’.

Subsequently, in the state where the luminescent color is enhanced asshown in FIG. 4, the entire video signal is compressed. The compressiongain is uniformly applied to pixels of the entire screen. Accordingly,the luminescent color is emphasized more than pixels of othernon-luminescent colors by an effect of the enhancement and a luminescentcolor part is allowed to be highlighted. That is, this processing isequal to redistribution of luminance on a screen. Moreover, an amount ofgain is changed according to a geometric average value and anenhancement is not carried out in a bright image, thus inhibiting theentire screen from becoming dark by compressing the bright image afterthe enhancement thereof.

The amount of the compression gain is defined based on prescribedconditions specifying brightness of a screen and the like. For example,when display is performed only by carrying out an enhancement, a screenaverage picture level (APL) of a video signal is increased. This isreturned to an original APL level by the compression gain, the APL ofthe entire screen is thereby maintained, and in addition, it is possibleto redistribute luminance so as to highlight the luminescent color bycarrying out the enhancement to compression gain processing. Moreover,relation between gain at the time of the enhancement and the compressiongain may be defined in advance.

FIG. 5 is a diagram showing an example of a state before and aftercarrying out the enhancement to the compression processing with a tonecurve. As described above, in the embodiment according to the presentinvention, a pixel with the CMI which is greater than or equal to the‘thresh’ is regarded as a luminescent color, and an enhancement iscarried out for the pixel. The enhancement is carried out for L* inCIELAB color space and then returned to RGB data. Subsequently, thecompression gain is applied to the entire video.

The tone curve shown in FIG. 5 shows tone characteristics of input andoutput of the RGB data, and is displayed with 0 to 255 tones with eightbits. Additionally, symbol ‘a’ indicates a state before the enhancementwhile symbol ‘b’ indicates a state after the enhancement and compressionprocessing. A luminescent color boundary is a tone corresponding to a‘thresh’ of a CMI, atone curve is more greatly inclined due to theenhancement in a tone which is greater than or equal to the luminescentcolor boundary, and an output tone exceeds 255 in the vicinity of thehighest tone (255). This is because a CMI greater than 1.0 is allowedwithout being clipped even in a case where a CMI becomes greater thanCMI 100 at the time of the enhancement as shown in FIG. 4. Even afterthe compression gain is applied thereto, the output tone exceeds 255 insome cases.

FIG. 6 is a diagram showing a tone curve with an RGBLUT created from RGBdata shown by the tone curve in FIG. 5. In the diagram, a MAX outputtone is detected from data after compression of FIG. 5. Then, the RGBLUTso as to output a 255 tone when a tone signal as with the MAX outputtone is input is created. At the time, the LUT is such that relationbetween input and output is linear as far as output is 0 to 255×N % (Nis a prescribed constant). A quadratic functionY(x)=A+B×(x−x1)+C×(x−x1)2 passing through (x1, y1)=(255×N, 255×N) and(x2, y2)=(MAX, 255) is then calculated. Conditions thereof are based on(1) y(x1)=y1 (linear LUT and coordinates are continuous), (2)y′(x1)=y1/x1 (linear LUT and an inclination are continuous), and (3)y(x2)=y2 (initial conditions). An obtained tone curve c is, as shown inFIG. 6, formed such that relation is linear where input and output havethe same values as far as input is 0 to 255×N %, an inclination ischanged when input is greater than or equal to 255×N %, and output is255 when input is MAX.

FIG. 7 is a diagram showing a tone curve (b) in a case where output ofthe tone curve (b) in FIG. 5 is used as input in FIG. 6. Accordingly, ina tone which is greater than or equal to the luminescent color boundary,a tone curve in a curved shape is formed toward (x, y)=(255, 255) sothat an output value does not exceed 255, thereby performing clip softlyto keep a tone as far as possible and reducing a sense of discomfort ondisplay. Note that, the above-described processing may be executed byoperation processing of software without using an LUT in which an outputvalue for input is prepared in advance.

As described above, in the luminescent color enhancement processingaccording to the present invention, a luminescent color which is abright color is detected from a CMI histogram of a video signal, and theluminescent color is enhanced when a geometric average value of thevideo signal is smaller than a prescribed value, that is, a video isdark. The video signal after the enhancement is then compressed toredistribute luminance. This makes it possible to perform high-qualityvideo display by highlighting a part in a luminescent color. The effectof the enhancement is great especially in a video with a dark inputvideo. Moreover, in a video having many luminescent color regions aswell as many parts in which a tone is saturated, it is possible to avoidthat a screen looks dark due to compression processing after theenhancement by not carrying out a luminescent color enhancement.

(CMI Detection Processing)

Next, description will be specifically given for detection processing ofthe CMI which is used in the above-described luminescent colorenhancement processing.

As described above, a CMI (Color Mode Index) is an index showing to whatextent a color of interest is bright. Here, the CMI is different fromluminance and shows brightness to which information of a color is added.The CMI is defined byL*/L*modeboundary×100  formula (1).

The above-described L* is an index of relative color brightness, andbecomes lightness of white which is the brightest as an object colorwhen L*=100. In the above-described formula (1), L* is lightness of acolor of interest, while L*modeboundary is lightness of a boundary whichlooks like emitting light in chromaticity as with the color of interest.Here, it is known that L*modeboundary≈lightness of an optimal color(brightest color among object colors). Lightness of a color in whichCMI=100 is referred to as a luminescent color boundary, and it isdefined as emitting light (being a luminescent color) in the case ofexceeding CMI=100. The luminescent color boundary where CMI=100 inrelation between saturation and lightness is schematically shown in FIG.8.

In the real world, objects with brightness exceeding that of thebrightest object color (optimal color) (objects emitting light) existand are allowed to be seen by a human. That is, there exist many colorsin which CMI>100. Here, for a broadcast signal or image data, aluminance range is compressed by a stop of a camera or the like at thetime of shooting, and moreover, standardized and made into data in acolor gamut according to standards such as NTSC or EBU. Accordingly,even a reflected color or a luminescent color is standardized, so that avideo signal is in a state of having no color in which the CMI exceeds100 (that is, exceeding lightness of the optimal color).

For example, a luminance histogram of a real object or the like is shownin FIG. 9(A) while a histogram after standardization is shown in FIG.9(B). In the diagrams, symbols ‘h1’ and ‘h2’ show luminance histogramsin two scenes of the real world, in which h1 is a luminance histogram ina relatively dark scene while h2 is a luminance histogram in arelatively bright scene. In the example of FIG. 9(A), the luminancehistogram of a real object or the like has brightness of 10,000 cd/m² atmaximum on h1 and brightness of 100 cd/m² at maximum on h2. When thescene of the real world is made into data by imaging or the like,histogram data is standardized as shown in FIG. 9(B). Then, video signaldata thereof has 255 tones in both cases with luminance of the object of10,000 cd/m² and that of 100 cd/m². In other words, the luminance rangeis compressed, and thus original luminance information is lost.

Based on the above-described circumstances, description will be givenfor a method of calculating a CMI from a broadcast video signal to bedisplayed on the video display device with reference to FIG. 10. Thebroadcast video signal is standardized based on the BT.709 standard tobe transmitted. Accordingly, first of all, RGB data of the broadcastvideo signal is converted into data of tristimulus values XYZ using atransformation matrix for the BT.709. Lightness L* is then calculatedfrom Y using transformation. It is assumed that L* of a color ofinterest was at a position P1 in FIG. 10. Next, chromaticity iscalculated from the converted XYZ to check L* of the optimal color withchromaticity as with the color of interest (L*modeboundary) from data ofthe optimal color which is already known. A position thereof on FIG. 10is P2.

The CMI is calculated from these values using the above-describedformula (1). The CMI is indicated by a ratio between L* of a pixel ofinterest and L* of the optimal color of chromaticity thereof(L*modeboundary).

The CMI is obtained for each pixel of a video signal by the method asdescribed above. The broadcast signal is standardized, and thus allpixels have any CMI in a range from 0 to 100. Then, a CMI histogram iscreated for a video of one frame with CMI on a horizontal axis and afrequency on a vertical axis. An example of the CMI histogram is shownin FIG. 11. An average A and a standard deviation σ are calculated fromthe created CMI histogram.

It is impossible to appropriately detect a pixel which is emitting lightbased on a certain absolute value calculated from the standardized videosignal. Accordingly, whether or not light is emitted is distinguishedwith relative distribution of a video signal of one frame. Here, astrikingly bright pixel in the entire video of one frame is defined as apixel which is emitting light. In this case, a CMI higher than theaverage A by Nσ (N is a prescribed constant) (A+Nσ) is defined as aluminescent color boundary, and a pixel having CMI which is greater thanor equal to the luminescent color boundary is regarded as emittinglight. A value of the constant N is not limited, however, N=2.8, forexample.

By specific processing as described above, a CMI histogram is generatedfrom a video signal, a ‘thresh’ is calculated from the histogram, andthereby making it possible to execute luminescent color enhancementprocessing as described above based on a geometric average value.

(Geometric Average Value Detection Processing)

Next, description will be given specifically for detection processing ofa geometric average value which is used in the above-describedluminescent color enhancement processing. The geometric average value(Geometric Average) is not an average of signal luminance but aluminance average value in which an average of luminance of a liquidcrystal panel is calculated as a value that is coincident with a visualperformance, and is specifically represented by the following formula(2).

[Formula I]

$\begin{matrix}{{{GeometricAve}.} = {\exp\left( {\frac{1}{n}{\sum\limits_{pixels}{\log\left( {\delta + Y_{hum}} \right)}}} \right)}} & {{formula}\mspace{14mu}(2)}\end{matrix}$

In the above-described formula (2), δ is a minute value which is notable to diverge calculation, and, for example, δ=0.00001. Additionally,Ylum indicates panel luminance, having a value of 0 to 1.0. The Ylum isallowed to be represented as (signal luminance/MAX luminance)^γ.Moreover, n indicates the number of pixels and “pixels” indicates thetotal number of pixels. In this way, in the formula (2), a logarithmicaverage of a luminance value of a pixel of a video is raised, in otherwords, a value of a geometric mean of luminance is indicated, and thevalue is greatly influenced by black.

FIG. 12 is a diagram showing a response curve of a human photoreceptorcell with respect to luminance. As shown in FIG. 12, the response curveof a human photoreceptor cell depends on a luminance value having alogarithm (luminance (log cd/m²)). This is generally referred to as theMickaelis-Menten Equation.

A geometric average value is obtained by raising a logarithmic averageof a luminance value of a pixel as described above, and thus thegeometric average value may be referred to as a value that response ofeyes to an image (that is, how bright the image looks) is quantified.That is, it may be said that the geometric average value is close to ahuman sensory amount, and this value is used as a video feature value todetermine in the CMI histogram whether or not luminescent colorenhancement processing is performed.

FIG. 13 is a flowchart for explaining luminescent color enhancementprocessing in a first embodiment of the video display device accordingto the present invention, showing a flow of luminescent colorenhancement processing for one frame of a video signal.

First, a CMI histogram is created from an input video signal (step S1).The CMI histogram is obtained from a ratio between L* and L* of theoptimal color (L:modeboundary) of each pixel. A geometric average valueis then calculated from the CMI histogram (step S2). Subsequently, a‘thresh’ for deciding a CMI region in which a luminescent colorenhancement is carried out is calculated from the CMI histogram (stepS3). The ‘thresh’ is calculated from an average value and a standarddeviation of the histogram. An order of calculating the geometricaverage value and calculating the ‘thresh’ may be reversed.

Next, based on the calculated geometric average value, whether or notthe luminescent color enhancement is carried out for the video of theframe is judged (step S4). When the video has a geometric average valuewhich is greater than or equal to a prescribed level, the luminescentcolor enhancement is not carried out. In a case where the luminescentcolor enhancement is carried out, for example, by calculating gain suchthat a pixel at a 2% position from a maximum CMI of the histogram hasCMI=100, and applying the gain to a pixel which is greater than or equalto the ‘thresh’, the luminescent color enhancement is carried out (stepS5). With the luminescent color enhancement, a tone of a pixel regardedas a luminescent color is emphasized.

Next, compression gain is calculated (step S6). The compression gain isdefined based on prescribed conditions to specify brightness of a screenand the like, or defined in relation to enhancing gain in advance. Thecalculated compression gain is then applied to the entire video tocompress a tone (step S7). This compresses a tone of the entire videowhich is enhanced. Redistribution of luminance in a screen is performedfrom the enhancement to compression processing. That is, emphasis isfurther performed for a bright color part while a tone is lowered for adark color part.

Finally, soft clip processing is performed (step S8). Here, soft clip isperformed for a pixel exceeding a highest tone (255, for example) evenafter compression by using a quadratic function.

Second Embodiment

Next, description will be given for a second embodiment of the videodisplay device according to the present invention. In theabove-described first embodiment, the ‘thresh’ is calculated from theCMI histogram, and the luminescent color enhancement is carried out forthe pixel which is greater than or equal to the ‘thresh’. On the otherhand, in the present embodiment, an operation is performed by using anRGB histogram or a Y histogram without using the CMI histogram. RGB dataand luminance value Y data correspond to another example of a featurevalue of the present invention.

In the above-described first embodiment, the RGB data of the videosignal is converted into XYZ by using a determinant to calculatechromaticity, and a CMI is obtained by a ratio between the chromaticityand L* of the optimal color. In this case, an operation amount forcalculating the CMI is a considerable amount. While it may be said thatthe CMI is preferable for the luminescent color enhancement processingsince the CMI is a feature value that is coincident with a human visualperformance, in the second embodiment, the RGB histogram or the Yhistogram is used in place of the CMI for the purpose of simplifying theoperation amount.

The CMI is a value indicating how bright the color of the pixel ofinterest is, compared to the optimal color with chromaticity as with thepixel of interest as described above. On the other hand, in acombination of RGB, a case where two colors have same chromaticity isequal to that an RGB ratio is not changed. That is, processing ofperforming operation of the optimal color with same chromaticity in theCMI is processing of obtaining a combination of RGB when atone of RGBdata becomes the greatest in a case where a ratio of RGB data is notchanged and multiplied by a certain number.

For example, a pixel having RGB data with a tone as shown in FIG. 14(A)is assumed to be a pixel of interest. When multiplying RGB data of thepixel of interest by a certain number, as shown in FIG. 14 (B), a colorwhen any one of RGB is saturated first is the brightest color withchromaticity as with an original pixel. When a tone of the pixel ofinterest of the color which is saturated first (R in this case) is r1,and a tone of R of the optimal color is r2, a value similar to the CMIis allowed to be obtained byr1/r2×100  formula (3).A color which is saturated first when RGB is multiplied by a certainnumber is a color having the greatest tone among RGB of the pixel ofinterest.

In the present embodiment, a value by the formula (3) as described aboveis calculated for each pixel to create a histogram. With this histogram,processing as with the CMI histogram in the first embodiment isperformed. That is, an average value and a standard deviation arecalculated from the histogram by the formula (3) to decide a ‘thresh’.Then, a geometric average value is calculated from the histogram toenhance a pixel corresponding to data which is greater than the ‘thresh’according to the level of the geometric average value to be compressedthereafter. By such processing, it is possible to reduce a burden of theoperation more than the operation processing of the CMI, and to performprocessing by, for example, a general-purpose IC or the like, and thussimplification is performed on a circuit scale.

Further, in the case of focusing only on brightness of a pixel, it ispossible to extract a luminance Y signal from a video signal, calculatea ratio between luminance of a pixel of interest and the maximumluminance to create a similar histogram, and perform enhancementprocessing by processing similar to the above-described processing.However, the Y histogram, with no information related to color, isinconsistent with the above-described processing by the CMI or the RGBhistogram. Note that, in processing of creating the Y histogram to carryout an enhancement, effective enhancement processing is possible with asimple configuration without raising a major obstacle.

Note that, in the above-described embodiment, the histogram is createdbased on the feature value of the video signal and the enhancement iscarried out by video signal processing for a pixel of the top of aprescribed range thereof, however, in performing the enhancementprocessing, a backlight light source for illuminating a liquid crystalpanel may be controlled to control brightness on a display screen.

In this case, for example, in uniformly decreasing gain over the entirepixels of one frame, in addition to applying the compression gain to thevideo signal, luminance of the backlight may be uniformly reduced tolower screen luminance. At the time, the compression gain of the videosignal and the backlight may work in concert to realize lowering ofluminance as desired.

EXPLANATIONS OF LETTERS OR NUMERALS

1 . . . video signal processing portion; 2 . . . histogram generationportion; 3 . . . enhancement processing portion; and 4 . . . compressiongain processing portion.

The invention claimed is:
 1. A video display device for enhancing anddisplaying an input video signal based on a feature value of the inputvideo signal, comprising: a histogram generation portion for generatinga histogram of the feature value of the input video signal; anenhancement processing portion for regarding a top region in aprescribed range of the histogram generated by the histogram generationportion as a luminescent color and enhancing a pixel value of theluminescent color; and a compression gain processing portion foruniformly decreasing gain over the entire pixels of one frame subjectedto enhancement processing after the enhancement processing portionperforms the enhancement processing, wherein the histogram generationportion calculates, for each pixel of one frame of an input videosignal, the feature value of a pixel and a possible maximum of thefeature value of the pixel, calculates, for each pixel, a ratio of thefeature value of the pixel to the maximum of the feature value as anindex value of color brightness, and generates a histogram in which thenumber of pixels is integrated according to the index value, theenhancement processing portion regards a pixel in which the index valuein the histogram generated by the histogram generation portion isgreater than or equal to a prescribed threshold as the top region andcarries out an enhancement by giving gain-up for a pixel value in thetop region, the enhancement processing portion integrates, at the timeof enhancing the pixel which is greater than or equal to the threshold,the number of pixels from a pixel having a highest feature value in adirection of low feature value of the histogram, applies gain-up inwhich the M-th (M is a prescribed value) percentile pixel of the totalnumber of pixels is displayed in a displayable highest tone, andtemporarily permits that the pixel value exceeds the displayable highesttone by the applied gain-up, and the compression gain processing portionperforms, in a case where the pixel value exceeds the displayablehighest tone even after the compression gain processing portion appliesgain-down for the temporarily permitted pixel value, soft clip so thatthe pixel exceeding the highest tone is output in a tone within thehighest tone as well as that an output value in a region from thehighest tone up to a prescribed low tone gradually lowers in a curve. 2.The video display device as defined in claim 1, wherein the enhancementprocessing portion determines, the threshold ‘thresh’ by‘thresh’=A+Nσ(N is a constant) where the prescribed threshold is a‘thresh’, an average value of the index value of the histogram is A, anda standard deviation is σ.
 3. The video display device as defined inclaim 1, wherein the enhancement processing portion exponentiates alogarithmic average of a luminance value of a pixel of one frame of aninput video signal to calculate a geometric average value, and carriesout the enhancement by the gain-up in a case where the calculatedgeometric average value is lower than a prescribed value.
 4. The videodisplay device as defined in claim 3, wherein the enhancement processingportion sets a second prescribed value which is lower than theprescribed value of the geometric average value, gradually increasesgain which is applied as the enhancement in order from the prescribedvalue up to the second prescribed value in a region from the prescribedvalue to the second prescribed value, and keeps the gain constant in aregion where the geometric average value is lower than the secondprescribed value.
 5. The video display device as defined in claim 1,wherein the feature value of the pixel is luminance L* which isspecified by CIELAB color space.
 6. The video display device as definedin claim 1, wherein the feature value of the pixel is data including amaximum tone value among pixel RGB data.
 7. The video display device asdefined in claim 1, wherein the feature value of the pixel is aluminance value Y of a pixel.